1. Field
This invention relates, in general, to semiconductor memory devices and, more particularly, to a memory device having an open bit line structure and a method of sensing data stored in such a memory device.
2. Description
In general, semiconductor memories can be classified into two different bit line structures: one is a folded bit line structure, and the other is an open bit line structure. In the folded bit line structure, a true bit line and a complementary bit-line are formed in parallel with each other on a same side of a sense amplifier as each other. In contrast, in the open bit-line structure, a true bit line and a complementary bit line are formed on opposite sides of a sense amplifier.
FIG. 6A shows a portion of an embodiment of a memory device having the folded bit line structure. FIG. 6B shows an embodiment of a memory device having the open bit line structure.
Of these two structures, the open bit line structure has been the focus of greater interest recently than the folded bit line structure, because the former permits a greater number of memory cells to be fit within a given unit area than the latter.
FIG. 1 shows in greater detail a memory device 100 having the open bit line structure. Memory device 100 includes: memory blocks 110 and 120; sense amplifiers 130-1˜n; memory unit cells MC1˜n1˜m, each including an access transistor AT and a memory cell capacitor CC; bit lines BL1—j through BLn—j alternating and in parallel in memory block 110 and BL1—i through BLn_i alternating and in parallel in memory block 120; and word lines WL1—j through WLm_j and WL1—i through WLm_i. Each of the memory blocks 110 and 120 includes n*m memory cells MC1˜n1˜m located at intersections of word lines and bit lines, wherein n is the number of bit lines and m is the number of word lines. Although for illustration purposes FIG. 1 shows only two memory blocks 110 and 120, it is understood that memory device 100 may include many more memory blocks that are not shown in FIG. 1.
When a word line of memory block 110 is selected, then memory block 110 includes only true bit lines (sometimes denoted “BL1˜n”) and the memory block 120 has only complementary bit lines (sometimes denoted “/BL1˜n”). On the other hand, When a word line of memory block 120 is selected, the memory block 110 has only complementary bit lines and the memory block 120 has only true bit lines. So the commonly used notation of “BL” and “/BL” can change according to which memory block is selected.
Each of sense amplifiers 130-1˜p has two inputs. One input is connected to a bit line BLk_j (e.g., an odd “true” bit line) in first memory block 110, and the other input is connected to a bit line BLp_i (e.g., an even “complementary” bit line) in the second memory block 120. For example, sense amplifier 130-1 has a first input connected to bit line BL1—j in memory block 110, and a second input connected to bit line BL2—i in memory block 120. Sense amplifiers 130-1˜n each sense and amplify a voltage difference between the voltages on the two bit lines connected to its two inputs.
As shown in FIG. 1, there exists a parasitic capacitance, (Cbl), between the lines of each adjacent bit line pair, BLk_j and BLk+1_j. Meanwhile, as the demand for higher integration and larger density increases, the distance between adjacent bit lines continues to become smaller. So the parasitic capacitance Cbl between the adjacent bit lines becomes a larger portion of the total bit line loading. In the folded bit line structure, the coupling effect of the parasitic capacitance Cbl can be cancelled by forming a bit line pair that crosses each other. However, in the open bit line structure, because a true bit line and a complementary bit line which are sensed by the sense amplifier are formed in different memory blocks, it is not easy to cancel the coupling effect caused by the parasitic capacitance Cbl.
To understand this better, FIG. 2 shows a structure of vertical memory cells MC1˜n1˜m that may be employed in memory device 100 of the FIG. 1. Each of the vertical memory cells MC1˜n1˜m includes a vertical channel transistor 210 as an access transistor AT, and a capacitor 220 as a memory cell capacitor CC. Capacitor 220 comprises storage node 222 and plate node 224. FIG. 2 shows access transistor 210 having a gate electrode connected to word line WL1-j, one of a drain and source electrode (e.g., drain electrode) connected to bit line BL1, and the other of drain and source electrode (e.g., source electrode) connected to storage node 222 of memory cell capacitor 220.
As shown in FIG. 2, the parasitic capacitance Cbl is created between adjacent bit lines, as described above. As the parasitic capacitance Cbl increases, one or more problems can develop with respect to the proper operation of the memory device 100. One such problem will be explained now with respect to FIGS. 4A and 4B.
Assume that in FIGS. 1 and 2 MC11 stores data corresponding to a digital value “0,” and MC21 stores data corresponding to a digital value “1.” When it is time to read the data, all of the bit lines are pre-charged to a voltage level of VCC/2. Then, a word line (e.g., WL1—j) is activated to turn on the access transistors 210 connected to the activated word line. At this point, a charge-sharing operation occurs, e.g., between bit line BL1—j and the charge stored on the memory cell capacitor CC of MC11 (data=0), and between BL2—j and the charge stored on the memory cell capacitor CC of MC21 (data=1), etc. As a result of the charge sharing operation, the voltage on BL1—j drops below VCC/2, while the voltage on BL2—j rises above VCC/2, etc. Accordingly, a voltage difference, ΔVbk, is created between the voltage on each odd “true” bit line of memory block 110 and the voltage on the corresponding even “complementary” bit line of memory block 120 for each bit line pair. For example, ΔVb1 is the difference between the voltage of BL1˜j (true bit line “BL1”) after the charge sharing operation and the voltage VCC/2 pre-charged onto the reference bit line BL2—i (complementary bit line “/BL1”). This voltage ΔVbk is then sensed and amplified by the corresponding sense amplifier 130 to read the data stored in the corresponding memory cell, MCk1.
FIG. 5A shows the bit line voltage waveforms for the case of an exemplary memory device 100 during the charge sharing and amplifying operation, as described above, in a case where the parasitic capacitance Cbl between adjacent bit lines is assumed to be negligible. In the example of FIG. 5A, it is assumed that: 256 vertical channel access transistors AT of memory cells MCk1 are connected to one bit line, BL1—j, and the total capacitance loading and resistance of the bit line is 34 fF and 50 kΩ respectively. The capacitance of a memory cell capacitor CC is 10 fF.
In the case of FIG. 5A, as explained above, when word line WL1—j is activated then bit line BL1—j is pulled down by the charge-sharing operation between BL1—j and the charge stored on memory cell capacitor CC of MC11 (data=0), so that a voltage ΔVb1 is developed between BL1—j and BL2—i (i.e., /BL1_ which is sufficient for sense amplifier 130-1 to sense a “0” stored in MC11, and amplify the voltage ΔVb1 to produce a correct output.
Thus, memory device 100 operates properly when there is very little or no parasitic capacitance Cbl between adjacent bit lines.
However, now consider the case where there is significant parasitic capacitance Cbl between adjacent bit lines, e.g., between bit line BL1—j and adjacent bit line BL2—j. FIG. 5B shows the bit line voltage waveforms for the case of an exemplary memory device 100 during the charge sharing and amplifying operation, as described above, where there is a significant coupling capacitance Cbl between adjacent bit lines. In the example of FIG. 5B, it is assumed that: 256 vertical channel access transistors AT of memory cells MCk1 are connected to one bit line, BL1—j, and the total capacitance loading and resistance of the bit line is 34 fF and 50 kΩ respectively. The capacitance of a memory cell capacitor CC is 10 fF. Meanwhile, in the example of FIG. 5B it is assumed that parasitic capacitance Cbl is 10 fF, or about 30% of the total bit line capacitance.
In this case, when word line WL1—j is activated then bit line BL1—j is pulled down by the charge-sharing operation between BL1—j and the charge stored on memory cell capacitor CC of MC11 (data=0). However, due to the coupling effect of parasitic capacitor Cbl between BL1—j and BL2—j, the charge stored on memory cell capacitor CC of MC21 (data=1) is also coupled, through Cbl, to bit line BL1—j. As a result, the voltage ΔVbl between BL1—j and BL2—i (i.e., /BL1) is smaller than the threshold sensing voltage of the sense amplifier 130-1 (e.g., an offset voltage of an input stage of sense amplifier 130-1). Thus, sense amplifier 130-1 amplifies the voltages of BL1—j and BL2—i to Vcc and the Vss, respectively. That is, a “1” is incorrectly sensed as being stored in memory cell MC11, instead of the correct value of “0.” Therefore, the bit line data sensing operation fails.
Accordingly, it would be advantageous to provide a memory device having an open bit line structure that can accurately sense data despite the existence of significant coupling capacitance between adjacent bit lines. It would also be advantageous to provide a method of sensing data stored in such a memory device. Other and further objects and advantages will appear hereinafter.
The present invention comprises a memory device having an open bit line structure and a method of sensing data stored in such a memory device.
In one aspect of the invention, a memory device comprises: a plurality of memory blocks each comprising a set of n bit lines and a set of m word lines and a plurality of memory cells provided at intersections of the bit lines and the word lines; a plurality of sense amplifiers, each sense amplifier having a first input connected to a kth bit line of a first memory block and a second input connected to an pth bit line of a second memory block; and a plurality of decoupling capacitors, wherein one of the decoupling capacitors is connected between the first input of one of the sense amplifiers and a bit line of the second memory block adjacent to the pth bit line, and a second one of the decoupling capacitors is connected between the second input of the one sense amplifier and a bit line of the first memory block adjacent to the kth bit line.
In another aspect of the invention, a memory device has a plurality of memory blocks, a plurality of sense amplifiers, and a plurality of decoupling capacitors, each memory block including a plurality of memory cells connected to word lines and bit lines, wherein each bit line is connected to an input of a first one of the sense amplifiers and is coupled through one of the decoupling capacitors to an input of a second one of the sense amplifiers.
In a further aspect of the invention, a memory device comprises: a plurality of memory blocks, each memory block comprising, a plurality of bit lines, a plurality of word lines, and a plurality of memory cells provided at intersections of the bit lines and word lines; a plurality of decoupling capacitors; and a plurality of sense amplifiers, each sense amplifier having a first input and a second input, the first input being connected to a first bit line of a first one of the memory blocks and being coupled via one of the decoupling capacitors to a first bit line of a second one of the memory blocks, and the second input being connected to a second bit line of the second one of the memory blocks and being coupled via another one of the decoupling capacitors to a second bit line of the first one of the memory blocks
In yet another aspect of the invention, a method is provided for sensing bit-line data in a memory device comprising a plurality of memory blocks. The method comprises: activating a word line of a first memory block; receiving a first voltage at a first input of a sense amplifier connected to a first bit line of the first memory block; receiving a second voltage at a second input of the sense amplifier connected to a bit line of a second memory block and capacitively coupled to a second bit line of the first memory block, wherein the first bit line of the first memory block is adjacent to the second bit line of the first memory block; and sensing a data value stored in the memory cell connected to the activated word line and the first bit line of the first memory block based on a difference between the received first voltage and the received second voltage.
In still another aspect of the invention, a method is provided for sensing bit-line data in a memory device comprising a plurality of memory blocks. The method comprises: providing a precharge voltage on a first bit line of a first memory block and a bit line of a second memory block; activating a word line of the first memory block; transferring to the first bit line of the first memory block a data voltage stored in a storage capacitor of a memory cell connected to the activated word line and the first bit line; coupling a coupling voltage from a second bit line of the first memory block to the first bit line of first memory block; receiving a first voltage, comprising a sum of the precharge voltage, the data voltage, and the coupling voltage, at a first input of a sense amplifier connected to the first bit line of the first memory block; capacitively coupling the coupling voltage from the second bit line of the first memory block to the bit line of the second memory block; receiving a second voltage, comprising a sum of the precharge voltage and the coupling voltage, at a second input of the sense amplifier voltage connected to the bit line of the second memory block; and sensing a data value stored in the memory cell connected to the activated word line and the first bit line of the first memory block based on a difference between the first voltage and the second voltage.